1. Field of the Invention
The present invention relates to an angle demodulation apparatus and angle demodulation method.
2. Description of the Related Art
Direct conversion is known as a scheme of demodulating an FM (Frequency Modulation) modulation signal. An FM receiver which uses the direct conversion scheme has a structure as shown in, for example, FIG. 5.
In the FM receiver shown in FIG. 5, an FM modulation signal is received by an antenna 301 and is amplified by an RF (Radio Frequency) amplifier 302. A branching filter 303 sends the amplified FM modulation signal to first mixers 304I and 304Q. The FM modulation signal is mixed with a pair of first local oscillation signals having a phase difference of 90 degrees to be converted to a pair of base band signals. The first local oscillation signals are generated by a first local oscillator 314 and a first phase shifter 308. Note that the frequencies of the first local oscillation signals are set to the same frequency as the frequency of the carrier frequency of the received signal.
The base band signals have their harmonic components cut off by LPFs (Low Pass Filters) 305I and 305Q and are then amplified by amplifiers 306I and 306Q. The amplified base band signals are respectively mixed with a pair of second local oscillation signals having a phase difference of 90 degrees by second mixers 307I and 307Q. The second local oscillation signals are generated by a second local oscillator 315 and a second phase shifter 309 . The mixing-originated signals are added together by an adder 310, thereby yielding a single intermediate frequency signal.
The intermediate frequency signal is supplied to an FM detector 313 via a BPF (Band Pass Filter) 311 and IF (Intermediate Frequency) amplifier 312. The FM detector 313 detects the intermediate frequency signal and outputs an audio signal originated from the detection.
The direct conversion scheme can simplify the structure of the apparatus that demodulates an FM modulation signal. Unlike superheterodyne, the direct conversion scheme does not suffer interference by signals whose frequencies lie in the vicinity of the image frequency.
When the first mixers cause the secondary distortion of the disturbance or the secondary distortion of the first local oscillation signals in the FM receiver with the above-described structure shown in FIG. 5, however, the DC component that is contained in the secondary distortion is mixed with the base band signals. When the DC component mixed in each base band signal is removed by an HPF (High Pass Filter), the carrier component of the FM modulation signal converted to that base band signal is eliminated too. This results in inaccurate demodulation of an FM modulation signal.
A possible scheme of eliminating the DC component without sacrificing the precise demodulation of the FM modulation signal is disclosed in, for example, U.S. Pat. No. 4,944,025.
The scheme taught by U.S. Pat. No. 4,944,025 allows the first local oscillator to implement AFC (Automatic Frequency Control) using the voltage that is acquired by adding an offset voltage to a voltage obtained by detecting the intermediate frequency signal. This scheme can make the frequency of the first local oscillation signal offset by a predetermined amount from the carrier frequency of an FM modulation signal which is to be received. It is therefore possible to easily eliminate the DC component from the base band signal using an HPF without sacrificing the precision.
However, the scheme makes it difficult to adjust the circuit that adds the offset voltage to a voltage obtained by detecting the intermediate frequency signal. Further, the offset amount of frequency becomes unstable. Because the first and second local oscillators that independently generate signals of different frequencies, the operation of the FM receiver is likely to become unstable. What is more, the structure of the FM receiver is complicated or large, thus resulting in a cost increase.